1. Field of the Invention
The present invention relates to an improved optical fiber ribbon cable in which the stress experienced by fibers located along the outside edge of the cable is minimized to improve the attenuation of those fibers.
2. Related Art
Referring to FIG. 1, conventional optical fiber ribbon cables 10 includes one or more optical fiber ribbons 12 each including a plurality of optical fibers (usually twelve) 14 disposed in a plane and encapsulated with a polymer containing an ultraviolet curable resin 16. Surrounding the optical fiber ribbon(s) 12 is a plastic jacket 18 with an aramid yam 20, or the like, located in the space 22 between the ribbon 12 and the jacket 18.
As shown in FIG. 1, the space 22 between the optical fiber ribbon 12 and the jacket 18 is substantially uniform around the periphery of the ribbon. The problem with this conventional design is that the optical fibers on the outside edge ("edge fibers"), identified by reference numeral 24, experience excessive stress during the manufacturing, installation and use of the cable, as compared to the interior fibers, identified by reference numeral 26. Hence, the attenuation of the edge fibers increases, as compared to the neighboring optical fibers. The stresses are magnified at temperature extremes (e.g., -20 degrees C.).
As noted above, the stress to which the fibers may be subjected is generated during the manufacturing process, the installation of the cable and during the static use of the cable. For example, the step of extruding the outerjacket onto the optical fiber ribbon naturally generates stresses in the optical fibers and particularly on the edge fibers 24. One reason for this is that when the outer jacket is extruded, coating is applied in a melted state and then cooled resulting in constriction of the fibers, and particularly the edge fibers. In addition, during the handling of the cable, pressure is inevitably applied to the jacket. However, due to the fact that the edge fibers 24 are on the outside edge of the cable, they are generally subjected to greater stress than the interior fibers 26. When a shock force is applied to the center portion of the cable, it is absorbed by many of the interior fibers such that the pressure (stress) experienced by any one of the interior fibers 26 is relatively small. In contrast, when the same shock is applied to the edge of the cable, a single edge fiber 24 absorbs the shock (i-e., it is not absorbed to by the interior fibers 26) such that the pressure (stress) experienced by the edge fiber is relatively large. These same considerations come into play during use of the cable when an external force is applied.
Finally, when the cable is subjected to a substantial change in temperature, the expansion and contraction of the jacket can damage the fibers and particularly the edge fibers 24.